The transduction of light energy to electrical changes in photorecepor membranes is a fundamental problem. A giant photoreceptor from the barnacle B. eburneus is ideally suited for such an investigation because of its large size and variegated electrical behavior. The problem will be investigated with uni-cellular electrophysiological techniques such as membrane potential recording, membrane polarization techniques (voltage and current clamp) and ion-specific microelectrodes. Changes in these membrane parameters will be correlated with absorbance changes of single cells obtained by microspectrophotometric means.